Systems and methods for improved performance of fluidic and microfluidic systems

ABSTRACT

Systems and methods for improved flow properties in fluidic and microfluidic systems are disclosed. The system includes a microfluidic device having a first microchannel, a fluid reservoir having a working fluid and a pressurized gas, a pump in communication with the fluid reservoir to maintain a desired pressure of the pressurized gas, and a fluid-resistance element located within a fluid path between the fluid reservoir and the first microchannel. The fluid-resistance element includes a first fluidic resistance that is substantially larger than a second fluidic resistance associated with the first microchannel.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Non-Provisional ApplicationNo. 15/403,942, filed Jan. 11, 2017, now allowed, which is acontinuation of International Application No. PCT/US2015/040026, filedJul. 10, 2015, which claims priority to U.S. Provisional Application No.62/127,438, filed Mar. 3, 2015, and U.S. Provisional Application No.62/024,361, filed Jul. 14, 2014, each of which is herein incorporated byreference in its entirety.

GOVERNMENT SUPPORT

This invention was made with government support under grant no.W911NF-12-2-0036 awarded by U.S. Department of Defense, AdvancedResearch Projects Agency. The government has certain rights in theinvention.

TECHNICAL FIELD

The present invention relates to fluidic and microfluidic systems.Specifically, the invention relates to systems and methods for providingimproved flow properties in fluidic and microfluidic systems.

BACKGROUND

Microfluidic devices for cell culture typically need to be perfused withfluid media at an extremely low flow rate, such as between 30 μL/hr and5 mL/hr. Moreover, in some experiments, the media must be flowed atknown rates for several weeks. Additionally, many experiments need tensof channels to be able to explore a number of test conditions, in orderto gain statistically significant results. Therefore, the devices arepreferably tested simultaneously using the same setup. What is more, thefootprint of the perfusion system must be small enough to allow for theintegration of many devices (e.g., at least 12 devices) in a reasonablespace.

Some existing technologies attempt to solve fluid-flow issues usingrollers on a stepper motor to engage an elastic polymer with channels init. As the motor spins, it pushes the fluid through the cartridge andthe chips. This pumping scheme, however, cannot be sufficientlyminimized. Additionally, because the pumping mechanism must be engagedwith the cartridge, the pump-head must reside inside an incubated space.Finally, the microfluidic peristaltic pumping system relies on amaterial that must be resistant to frictional/shearing forces, have alow elastic modulus, be injection moldable or otherwise mass producible,be bondable (to allow creating microfluidic channels), be non-cytotoxic,and not absorb or substantially adsorb small molecules. To date, itappears that there is no such material that sufficiently meets all ofthese demands.

Other alternative pumping technologies are traditional peristaltic pumpsand syringe pumps. Traditional peristaltic pumps tend to be bulky, andthey require specialized tubing to be strung up through the pump beforeeach use. In turn, the tubing needed to attain the low flow-ratestypical of organs-on-chips is difficult to connectorize and assemblewith chips or cartridges. Syringe pumps are also bulky, and they provideno simple way to change out syringes after the entire syringe volume hasbeen discharged.

Further, accumulation of gas bubbles poses risks to microfluidic systemsand components thereof. Gas bubbles can have many detrimental effectswhen introduced into or formed inside of microfluidic channels. Forexample, large capillary forces that are characteristic of bubbleinterfaces confined to small dimensions can cause difficulty in removingbubbles from the microfluidic channels. In microfluidic devices thatinclude cells, gas bubbles can be especially detrimental for severaladditional reasons. For example, stagnant bubbles sitting on top of thecells can starve them of critical nutrients. Further, even when onlypassing by cells, bubbles can damage the cells due to high shear andcapillary forces (e.g., by delaminating the cell layer). Many methodshave been proposed for preventing bubbles from entering microfluidicdevices or being formed within the devices. However, it is verydifficult to completely prevent bubble-generation events orbubble-accumulation events within a microfluidic system. Thus, it isunlikely that bubbles will be fully eliminated over the duration of along-term experiment.

Aspects of the present invention provide new fluid-pumping systems andmethods for microfluidic devices that provide a consistent and reliableflow of media to the microchannels and/or provide systems and methodsfor preventing, inhibiting, or limiting damage caused by bubbles thatare generated or accumulated.

SUMMARY

According to aspects of the present invention, a system for monitoring abiological function associated with cells includes a microfluidicdevice, a fluid line, and a fluid-resistance element. The microfluidicdevice includes a first microchannel, a second microchannel, and amembrane located at an interface region between the first microchanneland the second microchannel. The membrane includes a first side facingtoward the first microchannel and a second side facing toward the secondmicrochannel. The first side includes the cells adhered thereto, and thesecond side may also include the same or different cells adheredthereto. The fluid line is for delivering a working fluid to or from thefirst microchannel from a fluid reservoir. The fluid-resistance elementis coupled to the fluid line, although it may also be coupled to afluid-output line leading away from the microfluidic device. Thefluid-resistance element includes a first fluidic resistance that issubstantially larger than a second fluidic resistance associated withthe first microchannel.

According to further aspects of the present invention, a device formonitoring a biological function associated with cells includes a body.The body includes a first microchannel, a second microchannel, and amembrane located at an interface region between the first microchanneland the second microchannel. The membrane includes a first side facingtoward the first microchannel and a second side facing toward the secondmicrochannel. The first side includes the cells adhered thereto. Thebody further defines an internal fluid-resistance element coupled to thefirst microchannel. The internal fluid-resistance element having a firstfluidic resistance that is substantially larger than a second fluidicresistance associated with the first microchannel.

According to further aspects of the present invention, a system formonitoring a biological function associated with cells includes amicrofluidic device, a fluid reservoir, a pump mechanism, and a fluidresistance element. The microfluidic device includes a firstmicrochannel, a second microchannel, and a membrane located at aninterface region between the first microchannel and the secondmicrochannel. The membrane includes a first side facing toward the firstmicrochannel and a second side facing toward the second microchannel,the first side having the cells adhered thereto. The fluid reservoirincludes a working fluid and a pressurized gas. The pump mechanism is incommunication with the fluid reservoir to maintain a desired pressure ofthe pressurized gas. The fluid-resistance element is located within afluid path between the fluid reservoir and the first microchannel. Thefluid-resistance element includes a first fluidic resistance that issubstantially larger than a second fluidic resistance associated withthe first microchannel.

According to further aspects of the present invention, a system formonitoring a biological function associated with cells includes amicrofluidic device, a fluid reservoir, a pressure regulator, and asensor. The microfluidic device includes a first microchannel, a secondmicrochannel, and a membrane located at an interface region between thefirst microchannel and the second microchannel. The membrane includes afirst side facing toward the first microchannel and a second side facingtoward the second microchannel. The first side includes the cellsadhered thereto. The fluid reservoir includes a working fluid and apressurized gas. The fluid reservoir is coupled to the firstmicrochannel via a fluid line. The pressure regulator is incommunication with the fluid reservoir to maintain a desired pressure ofthe pressurized gas. The pressurized gas causes the working fluid tomove through the first microchannel. The sensor is for monitoring theactivity of the pressure regulator to determine an increase in fluidicresistance within the fluid line or the first microchannel.

According to further aspects of the present invention, a system formonitoring a biological function associated with cells includes amicrofluidic device, a fluid reservoir, a volumetric pump, and a sensor.The microfluidic device includes a first microchannel, a secondmicrochannel, and a membrane located at an interface region between thefirst microchannel and the second microchannel. The membrane includes afirst side facing toward the first microchannel and a second side facingtoward the second microchannel. The first side includes the cellsadhered thereto. The fluid reservoir includes a working fluid and apressurized gas. The fluid reservoir is coupled to the firstmicrochannel via a fluid line. The volumetric pump is in communicationwith the fluid reservoir to cause a pressure on the pressurized gas. Thepressurized gas causes the working fluid to move through the firstmicrochannel. The sensor is located between the fluid reservoir and thevolumetric pump. The sensor is for monitoring the pressure of a gas inthe system. In response to the pressure being below a predeterminedvalue, the volumetric pump supplies gas to the fluid reservoir at afirst volumetric flow rate. In response to the pressure being at orabove the predetermined value, the volumetric pump supplies gas to thefluid reservoir at a second volumetric flow rate. The second volumetricflow rate is less than the first volumetric flow rate.

According to further aspects of the present invention, a system formonitoring a biological function associated with cells includes amicrofluidic device, a fluid reservoir, and a pressure source. Themicrofluidic device includes a first microchannel, a secondmicrochannel, and a membrane located at an interface region between thefirst microchannel and the second microchannel. The membrane includes afirst side facing toward the first microchannel and a second side facingtoward the second microchannel. The first side includes the cellsadhered thereto. The fluid reservoir includes a working fluid and apressurized gas. The fluid reservoir is coupled to the firstmicrochannel via a fluid line. The pressurized gas causes the workingfluid to move through the first microchannel. The pressure source incommunication with the fluid reservoir to provide a modulated pressureprofile, the modulated pressure profile including periodic pressureincreases to inhibit the accumulation of bubbles in the system.

According to further aspects of the present invention, a microfluidicsystem includes a microfluidic device, a fluid reservoir, a fluid inputline, and a fluid-resistance element. The microfluidic device includes afirst microchannel. The fluid line is for delivering a working fluid toor from the first microchannel from the fluid reservoir. Thefluid-resistance element is within the fluid line. The fluid-resistanceelement includes a first fluidic resistance that is substantially largerthan a second fluidic resistance associated with the first microchannel.

According to further aspects of the present invention, a microfluidicsystem includes a microfluidic device having a first microchannel, afluid reservoir having a working fluid and a pressurized gas, a pump incommunication with the fluid reservoir to maintain a desired pressure ofthe pressurized gas, and a fluid-resistance element located within afluid path between the fluid reservoir and the first microchannel. Thefluid-resistance element includes a first fluidic resistance that issubstantially larger than a second fluidic resistance associated withthe first microchannel.

According to yet further aspects of the present invention, a system formonitoring a biological function associated with cells includes amicrofluidic device, a pressure source, and a controller. Themicrofluidic device includes a microchannel. The microchannel includes asurface. The cells are adhered to the surface. The pressure source isconfigured to cause a working fluid to flow along a fluid path thatincludes the first microchannel of the microfluidic device. Thecontroller is coupled to the pressure source and causes the pressuresource to be operable in a normal operating mode and a flushing mode.The normal operating mode includes the working fluid flowing past thecells within the microchannel at a first flow rate. The flushing modeincludes the working fluid flowing past the cells within themicrochannel at a second flow rate that is higher than the first flowrate to move an undesired aggregation of bubbles from a first positionin the fluid path toward a second position along the fluid path. Thecontroller switches to the flushing mode in response to a predeterminedcondition.

According to still yet further aspects of the present invention, amethod for monitoring a biological function associated with living cellsincludes flowing a working fluid at known fluid-flow conditions along afluid path during a normal operating mode and, on a periodic basisduring the normal operating mode, automatically flushing the fluid pathat a flushing flow rate for a time period to remove at least a portionof an undesired aggregation from the fluid path. The fluid path includesa microchannel of a microfluidic device. The microchannel includes cellsdisposed therein.

Further methods of using these systems are disclosed below and in theclaims.

These and other capabilities of the invention, along with the inventionitself, will be more fully understood after a review of the followingfigures, detailed description, and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates an example organ-on-chip device that may be usedwith the fluid-pumping system of the present invention.

FIG. 1B illustrates a first and second microchannel of the organ-on-chipdevice of FIG. 1A.

FIG. 2 is a schematic representation of a microfluidic system having afluid-resistance element, according to aspects of the presentdisclosure.

FIG. 3A is a schematic representation of a fluid-resistance element thatis separate from the organ-on-chip device, according to aspects of thepresent disclosure.

FIG. 3B is a schematic representation of a fluid-resistance element thatis a part of the organ-on-chip device, according to aspects of thepresent disclosure.

FIG. 4 illustrates a cartridge assembly including a tubularfluid-resistance element, according to aspects of the presentdisclosure.

FIG. 5A illustrates an interface including a blunt-end tube that is usedin the microfluidic system.

FIG. 5B illustrates an interface including a sharpened-tip tube that isused in the microfluidic system.

FIG. 6 is a flow-rate correlation graph of an example microfluidicsystem, according to aspects of the present disclosure.

FIG. 7A illustrates a moveable-cap assembly having a pressure linedisposed therethrough, for use in the microfluidic system.

FIG. 7B illustrates a moveable-cap assembly having a pressure linedisposed within the reservoir for use in the microfluidic system.

FIG. 8A is a schematic diagram of a microfluidic system having twofluid-resistance elements, each operatively coupled to a correspondingmicrofluidic channel of a single organ-on-chip device.

FIG. 8B is a schematic diagram of a microfluidic system having twofluid-resistance elements, each operatively coupled to a microfluidicchannel of separate organ-on-chip devices.

FIG. 8C is a schematic diagram of a microfluidic system having onefluid-resistance element operatively coupled to multiple microfluidicchannels.

FIG. 9 illustrates a schematic diagram of a microfluidic system having avalve-monitoring component, according to aspects of the presentdisclosure.

FIG. 10 is an exemplary correlation graph of flow rate versus thepercentage of time that the pressure valve is open, according to aspectsof the present disclosure.

FIG. 11 illustrates a schematic diagram of a microfluidic system havinga pressure-differential monitoring mechanism, according to aspects ofthe present disclosure.

FIG. 12 illustrates a schematic diagram of a microfluidic system havinga pressure sensor and feedback loop, according to aspects of the presentdisclosure.

FIG. 13 is a graph of liquid flow rate over time in an exemplary systemhaving a continuous volumetric flow rate of supply gas.

FIG. 14 is a graph of flow-rate as pressure is increased for abubble-impinged system.

FIG. 15 illustrates an example pressure-modulation profile, according toaspects of the present disclosure.

FIG. 16 illustrates one embodiment of an exemplary system having apressure source disposed between the input reservoir and theorgan-on-chip device.

FIG. 17 illustrates an example flow profile having periodic flushingaccording to aspects of the present disclosure.

While the invention is susceptible to various modifications andalternative forms, specific embodiments have been shown by way ofexample in the drawings and will be described in detail herein. Itshould be understood, however, that the invention is not intended to belimited to the particular forms disclosed. Rather, the invention is tocover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION

While this invention is susceptible of embodiment in many differentforms, there is shown in the drawings and will herein be described indetail preferred embodiments of the invention with the understandingthat the present disclosure is to be considered as an exemplification ofthe principles of the invention and is not intended to limit the broadaspect of the invention to the embodiments illustrated. For purposes ofthe present detailed description, the singular includes the plural andvice versa (unless specifically disclaimed); the words “and” and “or”shall be both conjunctive and disjunctive; the word “all” means “any andall”; the word “any” means “any and all”; and the word “including” means“including without limitation.” Additionally, the singular terms “a,”“an,” and “the” include plural referents unless context clearlyindicates otherwise.

Aspects of the present invention relate to a pressure-driven system thatprovides smooth and reliable fluid flow through a microfluidic device ordevices. This pressure-driven system can utilize off-the-shelf,electronically controllable pressure regulators that can provide compactand long-lived actuation. Moreover, because the pumping action istransmitted by gas pressure, the modality enables designs wherein thebulk of the pumping system can remain both off the microfluidic deviceand off a cartridge that may hold one or more microfluidic devices,while not contacting biological liquids. This is an important advantage,because it can greatly simplify cartridge design, as well as reduceinstrument sterility or contamination concerns.

Additionally or alternatively, aspects of the present invention relateto systems and methods that prevent, inhibit, or limit damage caused bybubbles that form within the microfluidic system, providing forlong-term experimentation without significant loss, damage, ordegradation of components (e.g., cells) within the microfluidic device.Damage caused by bubbles is prevented, inhibited, or limited through useof automated fluid-flushing of the microfluidic devices usingperiodically increased flow rates. Surprisingly, automated periodicflushing is not only successful in removing accumulated bubbles from themicrofluidic system, but is also successful in preventing or inhibitingthe bubbles from growing, accumulating, or coalescing as they otherwisedo.

Another benefit of the periodic flushing is that, to the extent bubblesdevelop, they are smaller-sized bubbles, which lessens the opportunityfor the bubbles to damage microfluidic devices or become pinned furtherdownstream. This is especially beneficial in, for example, cellmicrofluidic cell-culture devices and organ-on-chip (“OOC”) devicesbecause smaller bubbles flowing through the microchannels reduce therisk of damage to the cells through, for example, delamination.Moreover, if a smaller bubble does become pinned to the surface of acell, it is less likely to cause the death of that cell becausenutrients may still be readily available to uncovered portions of thecell, or may be able to permeate to the cell through non-direct paths.Beneficially, periodic flushing allows for removal of bubbles withoutcausing damage to the cells or organ-level function of the OOC deviceand/or system.

The periodic flushing function of the system also may be used to delivera brief bolus of stimulus, drug, cytokine or nutrient or other molecularor chemical factor in physiological studies carried out using the OOCdevice 10.

FIGS. 1A and 1B illustrate one type of an OOC device 10. The OOC device10 includes a body 12 that is typically made of a polymeric material.The body 12 includes a first fluid inlet 14 a and a first fluid outlet14 b. The body 12 further includes a second fluid inlet 16 a and asecond fluid outlet 16 b. The first fluid inlet 14 a and the first fluidoutlet 14 b permit a fluid to passed through a first microchannel 24.The second fluid inlet 16 a and the second fluid outlet 16 b permitfluid to passed through a second microchannel 26. The first microchannel24 is separated from the second microchannel 26 by a membrane 30 at theinterface region. The membrane 30 may include a first cell layer 34 onone side and a second cell layer 36 on the opposing side. Often, thefirst cell layer 34 is one type of cell while the second cell layer 36is a second type of cell. The fluid moving through the firstmicrochannel 24 and exposed to the first cell layer 34 may be the sameas, or different from, the fluid moving through the second microchannel26 and exposed to the second cell layer 36.

Depending on the application, the membrane 30 may have a porosity topermit the migration of cells, particulates, proteins, chemicals and/ormedia between the first cell layer 32 and the second cell layer 36. Themembrane 30 is preferably flexible and elastic, allowing it to becontrollably stretched during use so as to understand the influence ofphysiological forces on the the first cell layer 32 and the second celllayer 36. As shown in FIG. 1A, the body 12 includes two ports 40 a and40 b that allow for a pressure differential within the body 12, therebycausing controlled movement of the membrane 30. More details on the OOCdevice 10 can be found in U.S. Pat. No. 8,647,861, which is owned by theassignee of the present application and is incorporated by reference inits entirety.

The fluidic resistance of the OOC device 10 is typically very low. Forexample, a pressure of less than 5 Pa is sufficient to generate thedesired perfusion flow rates. This pressure is very difficult to workwith, as it is easily overwhelmed, for example, by changes inhydrostatic head caused by fluid moving from an input reservoir to anoutput reservoir. For example, 1 cm of fluid-height change correspondsto around 100 Pa of pressure change.

The theory relating flow rate to fluidic resistance and pressuredifferential comes from the Euler Principle:

ΔP=R·Q

where ΔP is the differential pressure, Q is the flow rate, and R is thefluidic resistance. In other words, the fluidic resistance is theparameter that determines how much flow resistance when a pressure isapplied (under a given pressure, higher resistance yields a lower flowrate). Euler's Principle applies under laminar flow conditions andpredicts a linear relation between the three parameters.

In order to make the fluid-pumping system more robust, less sensitive tothe fluid levels in the reservoirs, and easier to control, pressures of500 Pa and up are preferable. Accordingly, the system increases thefluidic resistance of system to be much to be higher than that of theOOC device 10 alone. To accomplish this, systems in accordance with thepresent disclosure add one or more fluidic resistors in-line with theOOC device 10 to be perfused.

An additional advantage of using a resistor that dominates the flow'sfluidic resistance and is substantially higher than that of the OOCdevice 10 is that the flow rate is then less dependent on variations inthe chip's fluidic characteristics. In particular, flow rates are lesssusceptible to chip-to-chip or channel-to-channel variations (e.g. inmanufacture) or to changes during operation (e.g. due to cell growth orbubbles).

FIG. 2 is a schematic representation of a microfluidic system 100 formonitoring a biological function associated with cells that includes afluid-resistance element 102. The microfluidic system includes a pumpmechanism 104, an input fluid reservoir 106 a, and output fluidreservoir 106 b, a fluid-resistance element 102, and an OOC device 10.The pump mechanism 104 includes a pressure source 108 and a pressureregulator 110. The pressure source 108 supplies the system 100 withcompressed gases. The pressure regulator 110 controls the amount of thecompressed gases supplied to the system using, for example, a valve (notshown). In some embodiments, the valve is actuated between an open stateand a closed state. In the open state, the compressed gases are suppliedto the system 100. In the closed state, the valve inhibits thecompressed gases from being supplied to the system 100. In someembodiments, the valve includes a closed state and more than one openstate. The more than one open states provide a variable amount ofcompressed gases to the system, for example, the valve being 25% open,50% open, and the like.

The pump mechanism 104 is coupled to the input fluid reservoir 106 ausing a gas line 112. The input fluid reservoir 106 a contains a workingfluid 114 a and a pressurized gas 116 a. The working fluid 114 a is adesired fluid such as media or suspensions of cells, particulates,proteins, chemicals, combinations thereof, or the like. Or the workingfluid may expose the cells of the OOC device 10 to a contaminant,pollutant, or pharmaceutical to determine the how the cells react tosuch exposure. The pressurized gas 116 a applies a pressure to theworking fluid 114 a to move the working fluid 114 a out of the inputfluid reservoir 106 a and to the fluid-resistance element 102 and theOOC device 10.

Selection of a specific gas for use as the pressurized gas 116 a hassignificance because it may dissolve into the working fluid 114 a.Additionally, dissolving too much gas into the biological liquidincreases the risk of forming and accumulating bubbles within thedevice. This problem is exacerbated by higher applied pressures. Inorder to mitigate or eliminate this problem, the gas used to applypressure to the working fluid 114 a may be selected appropriately. Forexample, helium can be used as the pressurized gas 116 a, or as acomponent thereof, because helium's solubility in water is very low(assuming the working fluid 114 a is predominantly water). Additionallyor alternatively, the pressurized gas 116 a can be formulated accordingto the needs of the cell culture. For example, 5% CO₂ in air is atypical mixture. In some embodiments, where the nominal set-pressure is20 kPa, a mixture of about 20% helium and 80% the 5% CO₂-air mixture canbe used, so that the partial pressure of 5% CO₂-air mixture remains ator under what the partial pressure at atmospheric pressure.

The input fluid reservoir 106 a is coupled to the fluid-resistanceelement 102 using a fluid line 118. The fluid-resistance element 102provides a fluidic resistance to the working fluid 114 a that issubstantially larger than the fluid resistance associated with the OOCdevice 10. In some embodiments, the fluid resistance of thefluid-resistance element 102 is greater than about 10, 20, 100, 500,5000, or 50000 times the resistance of the OOC device 10. This fluidresistance results in a pressure drop or head loss of the working fluid114 a across the fluid-resistance element 102 prior to entering the OOCdevice 10. Beneficially, this provides for modularity of the system 100as fluid-resistance elements 102 can be interchanged to accommodatediffering operating conditions of the pump mechanisms 104 and OOCdevices 10. As will be described below with reference to FIGS. 3A-4, thefluid-resistance element 102 can be, for example, a channeled resistor302 or a tubular resistor 402.

The fluid-resistance element 102 is coupled to the OOC device 10 using afluid input line 120. As describe above with respect to FIG. 1, the OOCdevice 10 is configured to simulate a biological function associatedwith cells, such as simulated organs, tissues, etc. One or moreproperties of the working fluid 114 a may change as the working fluid114 a is passed through the microchannels 24, 26 of the OOC device 10 toproduce the effluent 114 b. As such, the effluent 114 b is still theworking fluid 114 a, but its properties and/or constituents may changewhen passing through the OOC device 10. The effluent 114 b is collectedby the output fluid reservoir 106 b for collection and later analysis.

FIG. 3A is a schematic representation of a channeled fluid-resistanceelement 302 that is separate from the OOC device 10. The channeledfluid-resistance element 302 includes a microfluidic channel 304 in asubstrate 306. The microfluidic channel 304 can be formed from a numberof known processes such as milling, blow-forming, hot embossing,injection molding, etc. The resistance provided by the channeledfluid-resistance element 302 can be selected by using the length and/orcross-sectional area of the microfluidic channel 304, as well as otherfeatures such as number of corners, number and size of restrictions andexpansions, cross-sectional shape, etc. For example, increasing thelength of the channel, decreasing the cross-sectional area, orincreasing the number of turns made by the channel will increase thefluidic resistance provided by the fluid-resistance element 302.Beneficially, channeled fluid-resistance elements 302 can be massproduced using known techniques to provide consistent resistancesbetween elements and, thus, systems.

The channeled fluid-resistance element 302 is coupled to the OOC device10 using fluid input line 120. The fluid input line 120 can be, forexample, capillary tubing, an interconnect adapter, or any suitabledevice to transmit the working fluid 114 a from the fluid resistanceelement 302 to the OOC device 10.

The channeled fluid-resistance element 302 can be configured to beremovably connected to, for example, a cartridge. The cartridge providesa user-friendly interface between the system and OOC device 10. In someembodiments, the cartridge removably receives the OOC device 10. In someembodiments, the fluid-resistance element 302 is removably attached tothe cartridge, and the OOC device 10 is removably coupled to thefluid-resistance element 302. This allows a variety of fluid-resistanceelements to be used with a single cartridge, thus reducing system costs.

Alternatively, the channeled fluid-resistance element 302 can beintegrally formed with the cartridge that is configured to hold one ormore OOC devices 10. Integrating the fluid-resistance element 302provides for a more user-friendly, monolithic cartridge.

FIG. 3B is a schematic representation of a channeled fluid-resistanceelement 302 that is a part of the organ-on-chip device 10′, according toaspects of the present disclosure. The OOC device 10′ includes anintegral channeled fluid-resistance element 302 and fluid input lineupstream from the first microchannel 24. Beneficially, incorporating thechanneled fluid-resistance element 302 on the OOC device 10 reduces thenumber of connections to be made within the system, thus reducing thepoints of failure and increasing user-friendliness.

FIG. 4 illustrates a cartridge assembly 400 having a tubularfluid-resistance element 402, according to aspects of the presentdisclosure. The cartridge assembly 400 includes housing having a topside 400 a and a bottom side 400 b that couple together to form theouter shell of the cartridge assembly 400. The top side 400 a includessystem-side input ports 404, chip-side input connectors 406, chip-sideoutput connectors 408, and system-side output ports 410. Eachsystem-side input port 404 is coupled to a respective chip-side inputconnector 406 using a length of capillary tubing 412. Each chip-sideoutput connector 408 is coupled to a respective system-side output port408 using a length of capillary tubing 412. The bottom side 400 bincludes a plurality of apertures configured to receive features such asthe chip-side inlet connectors 406, chip-side outlet connectors 408,alignment features, or fasteners therethrough.

When assembled, an OOC device 10 is coupled to the chip-side inputconnectors 406 and the chip-side output connectors 408 using, forexample, removable coupling. Beneficially, the capillary tubing 412 canbe used as the chip-side input connectors 406 and mate directly withports of the OOC device 10. The system-side input ports 404 receive theworking fluid 114 a from the system 100. The working fluid 114 a istransferred to the OOC device 10 using capillary tubing 412. This lengthof capillary tubing 412 provides the fluid-resistance element 402 byincluding several windings that increase the length of capillary tubing412 between the system-side input port 404 and the chip-side inputconnecter 406. This increased length provides a relatively largerpressure drop prior to the working fluid 114 a entering the OOC device10.

The cartridge assembly 400 can further include features to assist inexperimentation such as an illumination aperture 416. The illuminationapertures are disposed adjacent the OOC device 10 and allow light topass therethrough, providing for a variety of optical and spectroscopictechniques to be used.

Beneficially, the cartridge assembly 400 includes a modular design andprovides reconfigurable fluid resistance. For example, the fluidresistance can be altered by replacing the capillary tubing with tubinghaving a different diameter. Additionally or alternatively, the fluidresistance can be altered by replacing the capillary tubing with tubinghaving a different length. Capillary tubing 412 that is compatible withthe cartridge assembly 400 is widely available, and comes in a varietyof sizes and materials. Beneficially, an end-user may replace thecapillary tubing 412 to quickly, easily, and cheaply produce a cartridgeassembly 400 with a different fluid resistance. Additionally, the roundcross-sectional area of capillary tubing 412 is more advantageous forbubble clearance than non-round cross-sectional profiles.

As shown in FIG. 5A, a bubble 502 may accumulate or get trapped at aninterface 504, e.g., a tube-to-chip interface between, for example, thecartridge and the OOC device 10. This bubble accumulation may lowerperformance of the device, for example, by increasing fluidicresistance, or by dislodging and entering the cell-culture area. In someembodiments, this trapping or accumulation is reduced using a“sharpened” tip, for example, a cone. One example of a sharpened tip isshown in FIG. 5B. Surprisingly, this sharpened tip reduces bubbletrapping or accumulation at the port as compared to a blunt tip despiteincreasing both the hydrophobic surface area of the tip and the volumefor the bubbles to become trapped. It was discovered that this isbecause the bubble must maintain its fixed contact angle with thehydrophobic material, and it is energetically preferred that the bubbleis pushed off the surface, rather than the bubble bending its interfaceto wet the angled surface. This surprising result is more even morepronounced at an inlet. The tubing can either be manufactured with aconical or sharpened tip, or processed to provide such shapes aftermanufacture. This sharpened tip can also be beneficial at other pointsof the system, such as where the capillary tubing 412 meets thesystem-side output ports 410.

In some embodiments, the trapping or accumulation of a bubble is reducedusing hydrophilic surfaces. These surfaces are less likely to trap oraccumulate a bubble because they prefer to remain wetted by the aqueousliquid. The hydrophilic surface can be formed, for example, by formingthe nozzles from hydrophilic materials. Examples of hydrophilicmaterials that can be used are: glass, certain grades of polystyrene,polypropylene, or acrylic. Additionally or alternatively, the nozzlescan be treated to make them hydrophilic, for example, using a coatings,plasma treatment, etc.

FIG. 6 is a flow-rate correlation graph of an example microfluidicsystem 100 having a fluid-resistance element 102. The fluidic-resistanceelement 102 is a tubular resistor formed from polyetheretherketone(“PEEK”) tubing having an internal diameter of 100 μm, and a length of235 mm. As shown by the graph, the static pressure of the system isapproximately 0.2 kPa. The flow rate increases linearly with increasingsupply pressure. Beneficially, the fluidic-resistance element 102 allowssupply pressures in the order of Kilopascals to be used, while limitingachieved flow rates through the OOC device 10 on the order of a fewmicroliters per minute. For example, a supply pressure of 6 kPa can beused to deliver the working fluid 114 a to the OOC device 10 at a rateof about 6 μL/min.

Providing for the use of a relatively high supply pressure allows forcommercial “off-the-shelf” pumps and supplies to be used to drive thesystem. Additionally, disposing the fluidic-resistance element 102between the input reservoir 106 a and the OOC device 10 allows thedampening of flow fluctuations due to system effects such as regulatoror pump pulsatility. Beneficially, the fluidic-resistance element 102also makes it such that hydrostatic head, such as the head produced bythe height of liquid in the fluid reservoirs 106 a,b, is smallcontributor to the flow rate.

Referring again to FIG. 2, a number of pump mechanisms 104 may be usedin accordance with the present disclosure. These pump mechanisms 104 caninclude pressure-driven configurations or displacement-drivenconfigurations. In some embodiments, the pressure-driven configurationincludes a pressure regulator. The pressure regulator can be set toapply a desired pressure onto the working fluid 114 a. The appliedpressure generally corresponds to the desired flow rate through the OOCdevice 10. Beneficially, electrically controlled pressure regulatorssuch as the SMC ITV line of regulators, manufactured by SMC Corporation(Tokyo, Japan), enable control of the pressure and, indirectly, flowrate using, for example, a computer.

In some embodiments, the displacement-driven configuration includesvolumetric pumps such as a syringe pump or a two-port pump such as aperistaltic pump, piezo pump, braille pump, etc. The volumetric pumpsare configured to deliver a predetermined volumetric flow of gas to theinput reservoir 106 a. Unfortunately, delivering the gas at a constantvolumetric flow may require long periods of time for the system to buildup the needed pressure because the volumetric flow of operation will berelatively low. In some embodiments, the volumetric pump and/or othercomponents of the system are filled with a substantially incompressiblefluid. This reduces the gas volume, thus requiring less action to buildup the needed pressure. Additionally or alternatively, as will bedescribed below with respect to FIGS. 12-13, a pressure sensor can beused to monitor the generated pressure, and the pump can be set to afirst volumetric flow rate that is higher than the second, operatingflow rate in order to build the pressure to the predetermined set point.Advantageously, use of two-port pumps does not require the reciprocationthat the syringe pump requires and, thus, the two-port pump may continuewithout interruption.

Surprisingly, pressure-driven systems can be modified to allow for organlinking e.g., interconnecting multiple OOC devices 10 together inseries. This is because the canonical pressure-driven system is a“one-port pump”, e.g. lacking separate input and output ports like aperistaltic pump. Beneficially, a moveable cap, which normally seals thefluid reservoir 106 a and opens as needed to allow, for example, anautosampler to access the fluid reservoir. More details on theautosampler can be found in International Patent Application No.PCT/US14/46439, filed Jul. 11, 2014, which is owned by the assignee ofthe present application and is incorporated by reference in itsentirety.

FIG. 7A illustrates a moveable-cap assembly 700 having a pressure line702 disposed therethrough, according to aspects of the presentdisclosure. The movable-cap assembly 700 includes a cap member 704configured to pivot about a hinge 706. The cap member includes a gasket708 configured to form a substantially air-tight seal with the fluidreservoir 106 a. When not receiving a sample of media, the movable-capassembly 700 is sealed to the input fluid reservoir 106 a, and the inputfluid reservoir 106 a is pressurized by the pressure line 702. Whenreceiving a sample, the cap member 704 pivots about the hinge 706 torelease the seal of the fluid reservoir 106 a. After the sample isdeposited, the cap member 704 pivots about the hinge 706 to seal thefluid reservoir 106 a once again. The pressure line 702 then suppliespressure to the input fluid reservoir 106 a until the input fluidreservoir 106 a reaches a desired pressure. Beneficially, the pressureline 702 being disposed through the cap member provides for themovable-cap assembly 700 and pressure line 702 to be part of theautosampler or other instrument that interfaces with the cartridge,rather than the cartridge itself. This simplifies the design of thecartridge, as well as providing a convenient cartridge-to-systeminterface requiring fewer connections. FIG. 7B illustrates amoveable-cap assembly 700′ having a pressure line disposed 702′withinthe reservoir input reservoir 106 a, according to aspects of the presentdisclosure.

FIG. 8A is a schematic diagram of a microfluidic system 800 a having twofluid-resistance elements 102, each operatively coupled to a respectivemicrofluidic channel of a single OOC device 10. In this embodiment, thepressurized gas 116 a applies pressure to the working fluid 114 a. Theworking fluid 114 a is transferred to the fluid-resistance elements 102through a respective fluid line 118. Each fluid-resistance element 102is coupled to a respective channel of the OOC device 10 using afluid-input line 120. It is contemplated that the fluid-resistanceelements 102 can be selected to have different fluidic resistances,thereby resulting in different flow rates through the microfluidicchannels.

FIG. 8B is a schematic diagram of a microfluidic system 800 b having twofluid-resistance elements 102, each operatively coupled to a respectivemicrofluidic channel of separate organ-on-chip devices 10. The workingfluid 114 a is transferred to the fluid-resistance elements 102 througha respective fluid line 118. Each fluid-resistance element 102 iscoupled to a respective channel of a respective OOC device 10 using afluid-input line 120. It is contemplated that the fluid-resistanceelements 102 can be selected to have different fluidic resistances,thereby resulting in different flow rates through the different OOCdevices 10.

FIG. 8C is a schematic diagram of a microfluidic system 800 c having onefluid-resistance element 102 operatively coupled to multiplemicrofluidic channels. As shown, the multiple microfluidic channels maybe included on one or more OOC devices 10. The working fluid 114 a istransferred to the fluid-resistance element 102 through a fluid line118. The fluid-resistance element 102 is coupled to two channels of afirst OOC device 10 using a respective fluid-input line 120, and iscoupled to a channel of a second OOC device 10 using a fluid-input line120. Beneficially, use of several OOC devices 10 in parallel with onlyone pressure regulator leads to better data by helping to ensure the OOCdevices 10 or microfluidic channels 24, 26 are exposed to the sameconditions.

While a number of measures are taken to inhibit the bubble accumulationand trapping within microfluidic systems, it is generally impossible tocompletely prevent the accumulation of bubbles over the duration oftypical experiments, especially upon initial filling of the system.Flushing cycles, or periods of pumping at higher rates or pressures, canbe used in order to remove bubbles that have become trapped in thesystem. However, this flushing has potential drawbacks. First, each ofthe OOC devices 10 has a pumping rate that is defined by shear rates andother physiologically relevant parameters that limit the duration andintensity of a flushing cycle. Additionally, the longer the flushingcycle, the more media is pumped through the microfluidic system, whichundesirably leads to a more dilute effluent that makes quantitativechemical and biological readouts more difficult. To address thesedrawbacks, systems in accord with the present disclosure may includemonitoring mechanisms to determine a general flow rate throughout thesystem. From this, an accumulation of bubbles can be detected and aflushing cycle can be selectively actuated.

FIG. 9 illustrates a schematic diagram of a microfluidic system having avalve-monitoring component 902. Pressure regulators 110 use a pressuresource 108 having a relatively high-pressure gas, and transferquantities of the high-pressure gas to the output (e.g., gas line 112)via a valve (not shown). In some embodiments, the valve is actuatedbetween an open state and a closed state, such as a solenoid valve. Theperiod of transfer of gas to the output results in the output having anew gas pressure that is lower than the relatively high-pressure gas. Insome embodiments, the valve includes a closed state and more than oneopen state, such as the valve being 25% open, 50% open, and the like.The level that the valve is open, e.g., 25% open, also affects the newgas pressure because the head loss across the valve is altered and, inturn, the quantity of gas transferred to the output is altered. Forexample, over a given time period, a valve that is 25% open will havegreater head loss and transfer less gas than a valve that is 50% open.

The valve-monitoring component 902 is coupled to the pressure regulator110 of the pump mechanism 104, and is configured to monitor one or moreproperties associated with the valve. In some embodiments, the one ormore properties monitored includes the period of time between valveopenings, the percentage of time that the valve is opened, the amountthat the valve is opened, combinations thereof, and the like. Thismonitoring can be done in a variety of ways, such as monitoring theelectronic control signal sent to the valve. These properties can thenbe associated with an estimated flow-rate of the system. For example,relatively higher liquid flow rates require the pressure reservoir to bereplenished more frequently (or with higher gas flow), which causes thevalve to open more. Beneficially, the relationship between flow rate andthe monitored parameter does not require a simple relationship, such asa linear relationship, but rather any monotonic relation between can beaccounted for through use of, for example a lookup table.

FIG. 10 is an exemplary correlation graph of flow rate versus thepercentage of time that the pressure valve is open. In this embodiment,the time that the valve remains open is monitored and is compared to thetotal time monitored. As shown, the percentage of time that the valve isopen has a linear relationship with the fluid flow rate through thesystem. While the illustrated correlation can provide a generalquantitative result, a quantitative result is not necessary for thismethod to be of value. For example, in a system including multiple OOCdevices 10, one of the OOC devices 10 may become blocked by air bubblesor debris and decrease the overall flow rate of the system.Beneficially, only modest fidelity is needed to monitor the controlsignal because the blockage of one OOC device 10 would cause asignificant reduction in flow rate, e.g., by one-third. In someembodiments, the blockage of one OOC device 10 in a group of three hasbeen detected in systems having a minimal flow rate (e.g., 1 μL/minnominal flow rate) through each OOC device 10.

Beneficially, the sensitivity of the sensor can be increased by addingone or more elements with high fluidic resistance between the pressureregulator 110 and the input reservoir 106 a. By adding the one or moreelements with high fluidic resistance, the valve remains open for longerperiods of time and/or opens more frequently. These increases increasethe sensitivity of detection without having to increase the fidelity orsampling time of the valve-monitoring component 902. In someembodiments, the high fluidic resistance element is a filter, which isdescribed with reference to FIG. 11 below.

FIG. 11 illustrates a schematic diagram of a microfluidic system 1100having a pressure-differential monitoring mechanism, according toaspects of the present disclosure. The differential monitoring mechanismincludes a pressure-differential monitoring mechanism 1102 disposedabout an element of high resistance 1104. As shown, the element of highresistance 1104 is a gas filter, which is often used to increase thepurity of the desired gas that comes in contact with the working fluid114 a. The pressure-differential monitoring mechanism 1102 monitors apressure drop across the high-fluidic-resistance element 1104. Becausefactors such as the supplied pressure and the filter resistance aregenerally known, the pressure drop can be correlated to flow in thesystem 1100. Beneficially, the higher operating pressure of the pumpmechanism 104 provides for use of readily availablepressure-differential monitoring mechanisms 1102 that cannot be usedwith typical microfluidic systems. In some embodiments, the fidelity ofthe pressure-differential monitoring mechanism 1102 is about 5 kPa, 1kPa, or 0.5 kPa. Beneficially, high-fluidic-resistance elements 1104such as filters are included in microfluidic systems to ensuresterility. These existing filter locations provide a convenient locationfor installing the pressure-differential monitoring mechanism 1102.

During operation, there is a slight, average pressure drop across theresistive element that might be measurable, depending on the fidelity ofthe pressure-differential monitoring mechanism 1102 used. If the entiresystem becomes clogged, fluid flow would stop and the pressure wouldequilibrate across the high-fluidic-resistance element 1104, resultingin zero differential pressure. Similarly, if one microfluidic channel inparallel with the one or more other channels becomes clogged, thepressure drop across the high-fluidic-resistance element 1104 woulddecrease as the total flow rate decreases. The measured differentialpressure may require some processing in order to relate to the liquidflow rate because the gas flow rate may be intermittent or uneven dueto, for example, discrete openings of the pressure value. In thisexample, an average or running average and a lookup table can be used todetermine a general liquid flow rate of the system.

FIG. 12 illustrates a schematic diagram of a microfluidic system 1200having a pressure sensor 1202 and feedback loop 1204. The pump mechanism104 includes a displacement-driven configuration that is used to delivera volume of gas to the input reservoir 106 a, thereby pressurizing theinput reservoir 106 a. The pressure sensor 1202 is disposed between thepump mechanism 104 and provides feedback on the pressure generated for agiven flow rate via the feedback loop 1204. Accordingly, both theflow-rate and pressure are directly observable in this system.

The pressure required to attain a particular flow rate within the system1200 can be predicted with some degree of accuracy. Thus, so long as thesystem is operating nominally, the flow rate and pressure should fall inline with each other. Because both quantities are directly observable inthis system, this condition is easily verified, and any deviation fromthe pressure/flow-rate relation could indicate an obstruction, leak, orother system failure. This is a very powerful source of feedback, and itcould be used to detect or diagnose a variety of conditions with highaccuracy. Beneficially, the system can be actuated to more-selectivelyremedy these system failures. In some embodiments, the volumetric pumpmoves only when the system pressure falls below a predeterminedthreshold. The volumetric pump can be monitored for movement todetermine whether a blockage has occurred. A blockage may be indicatedby the measured system pressure remaining generally constant, but thepump not moving for a period of time. To remove the blockage, thepredetermined threshold may be raised to a second, higher pressure toincrease the system pressure and clear the blockage.

FIG. 13 is a graph of liquid flow rate over time in an exemplary systemhaving a continuous volumetric flow rate of supply gas. To generate thegraph, a syringe pump set at a constant rate of 2 μL/min was used tosupply gas to the system. As shown, at this constant flow rate, it tookapproximately two hours for the liquid flow rate to reach 90% of the gasflow rate. Beneficially, this time can be dramatically reduced bysetting the syringe pump to a first, higher volumetric flow rate, forexample 1 mL/min, and monitoring the pressure of the system. When thepressure reaches a first predetermined value, the syringe pump can bereduced to a second, lower volumetric flow rate, for example, 2 μL/min.In some embodiments, the second volumetric flow rate is approximatelyequal to the volumetric flow rate of the working fluid 114 a through thesystem. In some embodiments, the first volumetric flow rate is about 10times greater than the second volumetric flow rate. In some embodiments,the first volumetric flow rate is about 100 times greater than thesecond volumetric flow rate. In some embodiments, the first volumetricflow rate is about 1000 times greater than the second volumetric flowrate. In one example, the ramp-up time is reduced from about 2 hours toabout 1.5 minutes using the first and the second volumetric flow rates.Beneficially, such a reduction in ramp-up time reduces the amount of lagtime for experiments, as well as the quality of the experimental data.It is contemplated that more sophisticated control strategies may beused to set the syringe rates, for example,proportional-integral-derivative control, a subset thereof, or the like.

An additional problem that arises with bubble accumulation or debrisaccumulation is accumulation at inputs, outputs, or within components ofthe system can stop flow entirely and lead to critical, sometimesirreversible failure of the system. Moreover, without special action,the system may be unable to remove the obstruction and recover withoutspecial action. For example, the system may not include a set point thatis high enough to overcome the pinning action on the obstruction.

FIG. 14 is a graph of flow-rate as pressure is increased for abubble-impinged system. As is shown, the flow rate of the system remainszero until the pinning action is overcome at 3.85 kPa. After this point,the flow rate returns to the nominal flow rate expected for a pressureof 3.85 kPa. Thus, if the set point for the nominal flow rate of thesystem requires less than 3.85 kPa, the obstruction would never becleared and the system would fail.

Beneficially, the pressure applied to the system can followpressure-modulation profile that will assist in clearing accumulationsof bubbles and/or debris prior to those accumulations obstructing theentire system. Additionally or alternatively, a pressure-modulationprofile can be used that will assist in clearing an accumulations ofbubbles and/or debris that have obstructed the entire system. Thepressure-modulation profile applies a desired pressure for apredetermined amount of time, and then, periodically, increases theapplied pressure to a second desired pressure for a period of timebefore returning to the first desired pressure.

FIG. 15 illustrates an example pressure-modulation profile, according toaspects of the present disclosure. In the illustrated example, apressure of 1 kPa is applied to yield the desired nominal flow ratethrough the system. Periodically, the pressure is increased to apredetermined level for a short period of time. In the illustratedembodiment, the predetermined level of 10 kPa is used to clear anyobstructions from the system. The periodic cycle occurs about once everyminute, and the predetermined pressure is applied for about fiveseconds. Beneficially, use of pressure-modulation profiles allows forclearing obstructions from the system, without significantly increasingthe cost of the system with flow-rate monitors and the like. Moreover,use of pressure-modulation profiles allows for clearing obstructionsfrom the system without proactive action. Further, coordinatedmodulation of pressures in two microfluidic channels in an OOC device 10may be helpful where the channels are linked by a porous membrane.

FIG. 16 is a system 1600 including an input reservoir 106 a, a pressuresource 1602, an OOC device 10, and an output reservoir 106 b. In theillustrated embodiment, the pressure source 1602 is coupled to the fluidline between the input reservoir 106 a and the OOC device 10. Thepressure source 1602 is configured to increase the volumetric flow rateof the fluid into the OOC device 10. The pressure source 1602 is coupledto a controller (not shown) that controls operation of the pressuresource 1602. In some embodiments, the pressure source 1602 is a pumpsuch as a volumetric pump. The pressure source 1602 applies a force tomove the fluid from the upstream input reservoir 106 a toward the OOCdevice 10. In some embodiments, the pump is in direct communication withthe working fluid. For example, a volumetric pump may be placed indirect communication with the fluid such that the pump acts on a lengthof capillary tubing containing the fluid to displace the fluid and movea volume of the fluid downstream. As the frequency that the pressure isapplied increases or decreases (e.g., increasing or decreasing ther.p.m. of a peristaltic pump), the flow rate similarly increases ordecreases.

FIG. 17 illustrates an example modulated flow profile having periodicflushing cycles in the system 1600 of FIG. 16 that are triggered atpredetermined times by the pressure source 1602. This modulated flowprofile can also be applied to other microfluidic systems mentionedabove in FIGS. 1-15. In the illustrated embodiment, the system 1600periodically transitions between a normal operating mode and a flushingmode. As shown, the flow rate of the fluid is increased whentransitioning from the normal operating mode to the flushing mode, andis decreased when transitioning from the flushing mode to the normaloperating mode.

In the illustrated embodiment of FIG. 17, the flow rate during thenormal operating mode α remains generally constant during theexperiment. At a predetermined first time X, the flow rate of the systemis increased from the normal operating flow rate α to the flushing flowrate β. This ramp-up in flow rate can be caused by, for example,increasing the speed of a pump or increasing the pressure driving thefluid. The system remains at the flushing flow rate β for apredetermined period of time, and then returns to the normal operatingmode at the normal operating flow rate α. The system then proceeds innormal operating mode until reaching a second predetermined time Y, atwhich point the flushing cycle is triggered again. In some embodiments,the flushing flow rate is at least ten times higher than the operatingflow rate. In some embodiments, the flushing flow rate at least 100times higher than the operating flow rate. In some embodiments, theflushing cycle is triggered at consistently spaced time intervals.

During operation in the normal condition, an undesired aggregation(e.g., bubbles) will form at a first point in the fluid path. The flowproperties of the fluid path may be such that the aggregation will notgrow only to a certain point, and will not grow to fully occlude thefluid path. The flushing cycle is configured to cause at least a portionof the undesired aggregation to move from the first position toward asecond, downstream position. Beneficially, movement of the undesiredaggregation through the fluid path will occur due to the flushing cycleeven if that aggregation is not detectable by, for example, sensors.

In some embodiments, the normal operating mode proceeds for betweenabout two and about eight hours. In some embodiments, the normaloperating mode proceeds for between about two hours and about six hours.In some embodiments, the normal operating mode proceeds for about fourhours. In some embodiments, the flushing cycle lasts for between aboutfive seconds and about 120 seconds. In some embodiments, the flushingcycle lasts for between about ten and about ninety seconds. In someembodiments, the flushing cycle lasts for between about 20 seconds andabout 75 seconds. For example, surprisingly, systems that operated inthe normal operating mode for four hours, then entered a flushing cyclethat lasted a duration of between twenty and seventy-five seconds beforereturning to the normal operating mode were able to consistentlymaintain the viability and function of lung-on-a-chip devices for morethan seven days of perfusion.

In some embodiments, flushing of adjacent fluid paths is triggeredsimultaneously to inhibit damage to components of the system. Forexample, the fluid paths for the first microchannel 24 and the secondmicrochannel 26 of the OOC device 10 can be flushed at the same timeeven though one of the fluid paths may not need to be flushed asfrequently. Beneficially, the simultaneous flushing reduces stress tothe membrane 30 between the microchannels by reducing or maintaining thepressure differential across the membrane 30 during the flushing cycle.

It is contemplated that the normal operating flow rate α may increase ordecrease by known amounts during operation depending on the particularexperimental design. This known increase and/or decrease during normaloperating mode can be accounted for, and characteristics such as theperiod between flushing cycles, the difference in flow rate between thenormal operating mode and the flushing mode, the duration of theflushing mode, combinations thereof, and/or the like can be altered toaccount for these known changes in the operating modes.

It is further contemplated that systems and methods in accord with thepresent invention may include sensors to monitor characteristicsassociated with the fluid line such as flow rate, pressures, etc. Forexample, a sensor can be used to directly or indirectly detect theexistence of a triggering condition such as a bubble or bubbles in thesystem. A control mechanism can then be used to trigger a flushing cyclethat is in addition to, or in place of, the periodic flushing inresponse to the triggering condition. In some embodiments, an opticalbubble sensor is used to directly determine development of a bubblewithin the flow path. In some embodiments, a flow-rate sensor is used todetect the presence of a bubble by detecting augmented flow resistance(e.g., by detecting an unexpected change in flow rate or pressure).

Measurement and analysis of flow characteristics and/or triggering ofadditional flushing cycles can be used to adjust the time period betweenflushing cycles, adjust the flow rate used during the flushing cycle,adjust the duration of the flushing cycle, etc. In some embodiments, thedetection of a bubble and initiation of a non-periodic flush will causethe system to reduce the time period between flushing cycles. In someembodiments, the detection of unexpected flow characteristics duringnormal operation mode will cause the system to increase the flow rateused during the flushing cycle or increase the duration of the flushingcycle.

While many of the above-described examples of increasing or decreasingflow through the system have been made with reference to increasing ordecreasing pump speed or driving pressures, it is contemplated thatvalves may be used in the system to provide control of flow rates. Forexample, valves, particularly microfluidic valves, can be used toconstrict or extend fluid paths, leading to lower or higher flow ratesof the working fluid.

While pressure-driven aspects of the present invention have beendescribed as applying pressure to push the working fluid 114 a throughthe system, in some embodiments, a vacuum is applied to the outputreservoir 106 b to pull the working fluid 114 a through the system. Insuch a case, the fluid-resistance element 102 still permits bettercontrol of the fluid flow as the working fluid is drawn to the inputreservoir. Further, this allows a wider range of pressurized gases to beused because the pressurized gas is in contact with the working fluid114 a after it has been in contact with the cells within the OOC device10. Moreover, if a vacuum is applied to the output reservoir 106 b incombination with pressurized gas 116 a applied to the input reservoir106 a, a wider range of pressurized gases can be used because the vacuumwill outgas the effluent 114 b.

While FIGS. 2, 9, 11, and 12 reference the output fluid reservoir 106 bhaving a gas 116 b at atmospheric pressure, it is contemplated thathigher pressures may also be used. Beneficially, the output reservoir106 b having the gas 116 b at a higher pressure provides for the systemto operate at a higher overall pressure. This higher operating pressureacross the system may provide beneficial properties, such as the systemmore closely replicating selected biological conditions, for examplebiological conditions of the kidneys.

While the input fluid reservoir 106 a and output fluid reservoir 106 bhave been described as storing the fluid in bulk, it is contemplatedthat capillary reservoirs may additionally or alternatively be used. Acapillary reservoir includes an elongated fluid path. The elongatedfluid path is configured to store the working fluid 114 a therein.Beneficially, this elongated fluid path can be configured to inhibitmixing of the working fluid 114 a between sample inputs. That is, theelongated fluid path beneficially retains time-based differences betweensamples injected in the reservoir at a first time, and samples injectedat a second time. This provides additional sensitivity to the datacollected by the system.

While FIGS. 2-4, 8A-9, and 11-12 depict the fluid-resistance element 102as disposed upstream from the OOC device 10, it is contemplated thatother configurations may be used. In some embodiments, the resistor isdisposed downstream from the OOC device 10. Beneficially, this canprovide for higher operating pressures for the OOC device 10. Thishigher operating pressure across the OOC device 10 may providebeneficial properties, such as the OOC device 10 more closelyreplicating selected biological conditions, for example, biologicalconditions of the kidneys. In some embodiments, fluid-resistanceelements 102 are disposed both upstream and downstream from the OOCdevice 10. Beneficially, disposing fluid-resistance elements 102 on boththe upstream and downstream of the OOC device 10 provide for a widerrange of pressures to be used at the input fluid reservoir 106 a and theoutput fluid reservoir 106 b.

FIGS. 9-15 have been described with reference to OOC devices 10 andmicrofluidic devices generally. However, the advantages of these systemsare not limited to microfluidic applications, but extend to other fluidsystems.

Beneficially, systems having pressure-driven flow can be easilymodified, or even used as-is, to also pump gas through the one or moremicrochannels on the OOC devices 10. This is useful, for example, forlung-on-chip airway channels, where the pushed gas can be used to clearthe airway channels of liquids or to perfuse it with air to simulatebreathing. In some embodiments, separate regulators are used, but sharea supply line, thus reducing the costs and complexity of the system.

According to some embodiments of the present invention, a system formonitoring a biological function associated with cells comprises:

-   -   (a) a microfluidic device having a first microchannel, a second        microchannel, and a membrane located at an interface region        between the first microchannel and the second microchannel, the        membrane including a first side facing toward the first        microchannel and a second side facing toward the second        microchannel, the first side having the cells adhered thereto;    -   (b) a fluid line for delivering a working fluid to or from the        first microchannel from or to, respectively, a fluid reservoir;        and    -   (c) a fluid-resistance element coupled to the fluid line, the        fluid-resistance element having a first fluidic resistance that        is substantially larger than a second fluidic resistance        associated with the first microchannel.

Optionally, the fluid reservoir includes the working fluid and apressurized gas. The pressurized gas the flow of the working fluidthrough the fluid line and fluid-resistance element.

Optionally, the fluid-resistance element includes a chip having anelongated fluid path. The first fluidic resistance is created by theelongated fluid path.

Optionally, the fluid-resistance element includes an elongated capillarytube has an elongated fluid path. The first fluidic resistance iscreated by the elongated fluid path.

Optionally, the capillary tube undergoes multiple windings within ahousing of the fluid-resistance element.

Optionally, a system or device may further include a pump mechanism toapply pressure to a gas within the fluid reservoir, thereby creating apressurized gas.

Optionally, the gas is substantially insoluble in the working fluid.

Optionally, the gas is a mixture of gases, the mixture including a gasthat is substantially insoluble in the working fluid.

Optionally, the first fluidic resistance is at least about 100 timesgreater than the second fluidic resistance.

Optionally, the fluid reservoir includes an elongated fluid path. Theelongated fluid path is configured to store the working fluid therein.

According to other embodiments of the present invention, a device formonitoring a biological function associated with cells comprises:

-   -   (a) a body having a first microchannel, a second microchannel,        and a membrane located at an interface region between the first        microchannel and the second microchannel, the membrane including        a first side facing toward the first microchannel and a second        side facing toward the second microchannel, the first side        having the cells adhered thereto;    -   (b) the body further defining an internal fluid-resistance        element coupled to the first microchannel, the internal        fluid-resistance element having a first fluidic resistance that        is substantially larger than a second fluidic resistance        associated with the first microchannel.

Optionally, the first fluidic resistance is at least about 100 timesgreater than the second fluidic resistance.

According to further embodiments of the present invention, a system formonitoring a biological function associated with cells comprises:

-   -   (c) a microfluidic device having a first microchannel, a second        microchannel, and a membrane located at an interface region        between the first microchannel and the second microchannel, the        membrane including a first side facing toward the first        microchannel and a second side facing toward the second        microchannel, the first side having the cells adhered thereto;    -   (d) a fluid reservoir having a working fluid and a pressurized        gas;    -   (e) a pump mechanism in communication with the fluid reservoir        to maintain a desired pressure of the pressurized gas; and    -   (f) a fluid-resistance element located within a fluid path        between the fluid reservoir and the first microchannel, the        fluid-resistance element having a first fluidic resistance that        is substantially larger than a second fluidic resistance        associated with the first microchannel.

Optionally, the fluid-resistance element includes a substrate having anelongated fluid path. The first fluidic resistance is created by theelongated fluid path.

Optionally, the fluid-resistance element includes an elongated capillarytube having an elongated fluid path. The first fluidic resistance iscreated by the elongated fluid path.

Optionally, the capillary tube undergoes multiple windings within ahousing of the fluid-resistance element.

Optionally, the system or device may further include a pressure sensorwithin the fluid reservoir, the pump mechanism being actuated inresponse to a predetermined output from the pressure sensor.

Optionally, the fluid resistance element is located upstream of thefirst microchannel.

Optionally, the fluid resistance element is located downstream from thefirst microchannel.

Optionally, the first fluidic resistance is at least about 100 timesgreater than the second fluidic resistance.

According to yet another embodiment of the present invention, a systemfor monitoring a biological function associated with cells comprises:

-   -   (g) a microfluidic device having a first microchannel, a second        microchannel, and a membrane located at an interface region        between the first microchannel and the second microchannel, the        membrane including a first side facing toward the first        microchannel and a second side facing toward the second        microchannel, the first side having the cells adhered thereto;    -   (h) a fluid reservoir having a working fluid and a pressurized        gas, the fluid reservoir being coupled to the first microchannel        via a fluid line;    -   (i) a pressure regulator in communication with the fluid        reservoir to maintain a desired pressure of the pressurized gas,        the pressurized gas causing the working fluid to move through        the first microchannel; and    -   (j) a sensor for monitoring activity of the pressure regulator        to determine an increase in fluidic resistance within the fluid        line or the first microchannel.

Optionally, the system or device may further include a fluid-resistanceelement located within a fluid path between the fluid reservoir and thefirst microchannel. The fluid-resistance element has a first fluidicresistance that is substantially larger than a second fluidic resistanceassociated with the first microchannel.

Optionally, the sensor monitors one or more properties associated with avalve of the pressure regulator.

Optionally, the one or more properties is a percentage of time that thevalve is open.

Optionally, the sensor monitors a control signal sent to the valve.

Optionally, the system or device may further include a gas filterlocated in a gas line between the pressure regulator and the fluidreservoir. The sensor measures the pressure differential across the gasfilter.

Optionally, the increase in fluidic resistance is caused by bubbles inthe fluid line or the first microchannel.

Optionally, the increase in fluidic resistance is caused by a blockagein the fluid line or the first microchannel.

Optionally, the blockage is associated with cells in the firstmicrochannel.

Optionally, the pressure regulator, in response to the sensordetermining the increase in fluidic resistance, maintains a secondpressure of the pressurized gas, the second pressure being selected toclear a blockage.

According to other embodiments of the present invention, a system formonitoring a biological function associated with cells comprises:

-   -   (k) a microfluidic device having a first microchannel, a second        microchannel, and a membrane located at an interface region        between the first microchannel and the second microchannel, the        membrane including a first side facing toward the first        microchannel and a second side facing toward the second        microchannel, the first side having the cells adhered thereto;    -   (l) a fluid reservoir having a working fluid and a pressurized        gas, the fluid reservoir being coupled to the first microchannel        via a fluid line;    -   (m) a volumetric pump in communication with the fluid reservoir        to cause a pressure on the pressurized gas, the pressurized gas        causing the working fluid to move through the first        microchannel; and    -   (n) a sensor located between the fluid reservoir and the        volumetric pump, the sensor for monitoring the pressure of a gas        in the system.

Optionally, in response to the pressure being below a predeterminedvalue, the volumetric pump supplies gas to the fluid reservoir at afirst volumetric flow rate. And wherein, in response to the pressurebeing at or above the predetermined value. The volumetric pump suppliesgas to the fluid reservoir at a second volumetric flow rate. The secondvolumetric flow rate is less than the first volumetric flow rate.

Optionally, the second volumetric flow rate is approximately equal to avolumetric flow rate of the working fluid through the firstmicrochannel.

Optionally, the first volumetric flow rate is at least about 100 timesgreater than the second volumetric flow rate.

According to yet other aspects of the present invention, a system formonitoring a biological function associated with cells comprises:

-   -   (o) a microfluidic device having a first microchannel, a second        microchannel, and a membrane located at an interface region        between the first microchannel and the second microchannel, the        membrane including a first side facing toward the first        microchannel and a second side facing toward the second        microchannel, the first side having the cells adhered thereto;    -   (p) a fluid reservoir having a working fluid and a pressurized        gas, the fluid reservoir being coupled to the first microchannel        via a fluid line, the pressurized gas causing the working fluid        to move through the first microchannel;    -   (q) a pressure source in communication with the fluid reservoir        to provide a pressure-modulation profile, the        pressure-modulation profile including periodic pressure        increases to inhibit the accumulation of bubbles or debris in        the system.

According to another aspect, the present invention involves a method ofmonitoring a biological function in a device having a membrane locatedon an interface region between a first microchannel and a secondmicrochannel. A first side of the membrane faces the first microchannelreceiving a working fluid and has a first type of cells adhered thereto.The method comprises:

-   -   (r) in response to applying pressure to a gas within a fluid        reservoir containing the gas and the working fluid, moving the        working fluid through a fluid-resistance element; and    -   (s) after moving the working fluid through the fluid-resistance        element, transferring the working fluid into the first        microchannel.

Optionally, the fluid-resistance element has a first fluidic resistancethat is substantially larger than a second fluidic resistance associatedwith the first microchannel.

Optionally, the first fluidic resistance is at least 100 times greaterthan the second fluidic resistance.

Optionally, the fluid-resistance element is located within the device influid path prior to the first microchannel.

According to yet another aspect, the present invention also involves amethod of detecting a fluid blockage in a microfluidic device having amembrane located on an interface region between a first microchannel anda second microchannel. The first side of the membrane faces the firstmicrochannel receiving a working fluid and has a first type of cellsadhered thereto. The method comprises:

-   -   (t) in response to applying pressure to a gas within a fluid        reservoir contain the gas and the working fluid, moving the        working fluid though the first microchannel; and    -   (u) monitoring an activity of a pressure source that provides        pressure to the gas.

Optionally, the monitoring includes monitoring a valve that isassociated with the pressure source.

Optionally, the monitoring includes monitoring the pressure differentialacross a filter located in a gas line between the pressure regulator andthe fluid reservoir.

Optionally, the monitoring includes monitoring movement of a volumetricpump, the volumetric pump being configured to move when the pressure tothe gas is below a predetermined threshold.

Optionally, the method may further include raising, in response to thevolumetric pump not moving for a period of time, the predeterminedthreshold to a second predetermined threshold.

According to yet a further aspect, the present invention includes amethod of inhibiting an accumulation of bubbles within a microfluidicsystem having a device with a membrane located at an interface regionbetween a first microchannel and a second microchannel. The first sideof the membrane faces the first microchannel receiving a working fluidand has a first type of cells adhered thereto. The microfluidic systemincludes a fluid line with a fluid-resistance element leading to thefirst microchannel within the device. The method comprises:

-   -   (v) in response to applying pressure to a gas within a fluid        reservoir containing the gas and the working fluid, moving the        working fluid through the fluid-resistance element and into the        first microchannel; and    -   (w) periodically increasing the pressure to the gas within the        fluid reservoir to advance bubbles through the first        microchannel and the fluid line.

Optionally, the pressure is increased to a level known to remove bubblesfrom the system by opening a valve that is associated with the pressuresource.

Optionally, the periodic increase in pressure occurs at one cycle perminute.

According to other aspects of the present invention, a microfluidicsystem comprises:

-   -   (x) a microfluidic device having a first microchannel;    -   (y) a fluid reservoir;    -   (z) a fluid line for delivering a working fluid to or from the        first microchannel from the fluid reservoir; and    -   (aa) a fluid-resistance element within the fluid line, the        fluid-resistance element having a first fluidic resistance that        is substantially larger than a second fluidic resistance        associated with the first microchannel.

In yet another aspect of the present invention, a microfluidic systemcomprises:

-   -   (bb) a microfluidic device having a first microchannel;    -   (cc) a fluid reservoir having a working fluid and a pressurized        gas;    -   (dd) a pump in communication with the fluid reservoir to        maintain a desired pressure of the pressurized gas; and    -   (ee) a fluid-resistance element located within a fluid path        between the fluid reservoir and the first microchannel, the        fluid-resistance element having a first fluidic resistance that        is substantially larger than a second fluidic resistance        associated with the first microchannel.

According to yet a further aspect of the present invention, a fluidsystem comprises:

-   -   (ff) a device having a first channel for receiving a working        fluid;    -   (gg) an input-fluid reservoir for holding the working fluid;    -   (hh) an output-fluid reservoir for holding the working fluid        after the working fluid has passed through the device;    -   (ii) a fluid line for delivering the working fluid from the        input-fluid reservoir and to the output-fluid reservoir, the        device being coupled to the fluid line; and    -   (jj) a fluid-resistance element within the fluid line, the        fluid-resistance element having a first fluidic resistance that        is substantially larger than a second fluidic resistance        associated with the first channel.

According to some other configurations of the present invention, a fluidsystem comprises:

-   -   (kk) a device having a first channel for receiving a working        fluid;    -   (ll) a fluid reservoir for holding the working fluid and a        pressurized gas;    -   (mm) a fluid line coupling the fluid reservoir and the device;    -   (nn) a pump in communication with the fluid reservoir to        maintain a desired pressure of the pressurized gas so as to        transfer the working fluid through the device; and    -   (oo) a fluid-resistance element within the fluid line, the        fluid-resistance element having a first fluidic resistance that        is substantially larger than a second fluidic resistance        associated with the first channel.

According to yet a further aspect, the present invention is a method forinhibiting formation of an occlusion in a microfluidic system thatcomprises:

-   -   (pp) flowing a working fluid through the microfluidic system at        a first flow rate for a first time period, an undesired        aggregation of bubbles forming at a first position in the        microfluidic system during the first time period;    -   (qq) in response to the first time period ending, increasing a        flow rate of the working fluid to a second flow rate for a        second time period to cause at least a portion of the undesired        bubbles to move from the first position toward a second        position; and (rr) repeating the acts of flowing and increasing        to cause a periodic bubble flushing within the microfluidic        system.

Optionally, the method may further include:

-   -   (ss) measuring characteristics of the microfluidic system;    -   (tt) in response to the measuring, adjusting at least one of the        first time period and the second time period during the        repeating.

Optionally, the adjusted first time period is longer than the initialfirst time period.

Optionally, the increasing is caused by a pump in direct communicationwith the working fluid.

Optionally, the method may further include:

-   -   (uu) measuring characteristics of the microfluidic system; and    -   (vv) in response to the measuring, adjusting the second flow        rate during the repeating.

Optionally, the adjusted second flow rate is greater than the initialsecond flow rate.

Optionally, the method may further include:

-   -   (ww) measuring, during the first time period, characteristics of        the microfluidic system;    -   (xx) analyzing the characteristics of the microfluidic system        for an occurrence of a triggering condition;    -   (yy) in response to the triggering condition, causing the        working fluid to flow through the microfluidic system at a third        flow rate;    -   (zz) after the response to the triggering condition, returning        the flow of the working fluid through the microfluidic system to        the first flow rate.

Optionally, the acts of causing the working fluid to flow through themicrofluidic system at the third flow rate and the returning the workingfluid to the first flow rate do not affect the ending of the first timeperiod.

Optionally, the third flow rate is equal to the second flow rate.

Optionally, the triggering condition is the detection of a bubble.

Optionally, the undesired aggregation of bubbles at the first positionabuts at least one cell.

According to yet another aspect of the present invention, a system formonitoring a biological function associated with cells comprises:

-   -   (aaa) a microfluidic device having a microchannel including a        surface, the cells being adhered to the surface;    -   (bbb) a pressure source configured to cause a working fluid to        flow along a fluid path that includes the first microchannel of        the microfluidic device; and    -   (ccc) a controller coupled to the pressure source and causing        the pressure source to be operable in a normal operating mode        and a flushing mode, the normal operating mode including the        working fluid flowing past the cells within the microchannel at        a first flow rate, the flushing mode including the working fluid        flowing past the cells within the microchannel at a second flow        rate that is higher than the first flow rate to move an        undesired aggregation of bubbles from a first position in the        fluid path toward a second position along the fluid path, the        controller switching to the flushing mode in response to a        predetermined condition.

Optionally, the predetermined condition is a predetermined interval oftime.

Optionally, the system or device may further include a sensor formeasuring characteristics of the system, wherein the predeterminedinterval of time is adjusted in response to the measuredcharacteristics.

Optionally, the system or device may further include a sensor formeasuring characteristics of the system.

Optionally, the predetermined condition is in response to the sensordetecting a triggering event.

Optionally, the triggering event is sensing the undesired aggregation ofbubbles reaching a predetermined threshold.

Optionally, the second flow rate is adjusted in response to the measuredcharacteristics.

Optionally, the pressure source is a pump in direct communication withthe working fluid.

Optionally, the pump is a volumetric pump.

Optionally, the undesired aggregation of bubbles at the first positionabuts at least one cell.

According to another aspect, the present invention is a method formonitoring a biological function associated with living cells thatcomprises:

-   -   (ddd) flowing a working fluid at known fluid-flow conditions        along a fluid path including a microchannel of a microfluidic        device during a normal operating mode, the microchannel having        cells disposed therein; and    -   (eee) on a periodic basis during the normal operating mode,        automatically flushing the fluid path at a flushing flow rate        for a time period to remove at least a portion of an undesired        aggregation from the fluid path.

Optionally, the periodic basis is automatically flushing at consistentlyspaced time intervals.

Optionally, the method is performed for at least seven days.

Optionally, the automatic flushing inhibits cell death due to theundesired aggregation.

Optionally, the undesired aggregation is an accumulation of air bubbles.

Optionally, the flushing flow rate is higher than a first flow rateassociated with the normal operating mode.

Optionally, the undesired aggregation would not form an occlusion undercontinuous flowing of the working fluid in the normal operating mode.

Optionally, the method may further include:

-   -   (fff) detecting a presence of a bubble along the fluid path; and    -   (ggg) in response to the detection and independent of the        automatic flushing, flushing the fluid path at a second flushing        flow rate.

Optionally, the second flushing flow rate is different from the flushingflow rate.

Optionally, the automatic flushing is caused by a pump in directcommunication with the working fluid.

Optionally, the automatic flushing is caused by raising the height of afluid reservoir holding the fluid reservoir.

Optionally, the automatic flushing is caused by altering a valve withinthe fluid path.

According to some other aspects of the present invention, a system formonitoring a biological function associated with cells comprises:

-   -   (hhh) a microfluidic device having a microchannel;    -   (iii) a fluid line for delivering a working fluid from a fluid        reservoir to or from the microchannel; and    -   (jjj) a fluid-resistance element coupled to the fluid line, the        fluid-resistance element having a first fluidic resistance that        is substantially larger than a second fluidic resistance        associated with the first microchannel.

Optionally, the fluid reservoir includes the working fluid and apressurized gas. The pressurized gas forces the flow of the workingfluid through the fluid line and fluid-resistance element.

Optionally, the fluid-resistance element includes a chip having anelongated fluid path. The first fluidic resistance is created by theelongated fluid path.

Optionally, the fluid-resistance element includes an elongated capillarytube having an elongated fluid path. The first fluidic resistance iscreated by the elongated fluid path.

Optionally, the capillary tube undergoes multiple windings within ahousing of the fluid-resistance element.

Optionally, the system or device may further include a pump mechanism toapply pressure to a gas within the fluid reservoir, thereby creating apressurized gas.

Optionally, the gas is substantially insoluble in the working fluid.

Optionally, the gas is a mixture of gases. The mixture includes a gasthat is substantially insoluble in the working fluid.

Optionally, the first fluidic resistance is at least about 100 timesgreater than the second fluidic resistance.

Optionally, the fluid reservoir includes an elongated fluid path, theelongated fluid path being configured to store the working fluidtherein.

While the present invention is described with reference to one or moreparticular embodiments or configurations, those skilled in the art willrecognize that many changes may be made thereto without departing fromthe spirit and scope of the present invention. Each of these embodimentsor configurations, and obvious variations thereof, is contemplated asfalling within the spirit and scope of the invention. It is alsocontemplated that additional embodiments according to aspects of thepresent invention may combine any number of features from any of theembodiments or configurations described herein.

What is claimed is:
 1. A method of flow, comprising: a) providing: amicrofluidic device comprising a fluid-resistance element and a fluidreservoir connected to a microchannel, wherein the fluid-resistanceelement has a first fluidic resistance that is at least about 100 timesgreater than a second fluidic resistance associated with themicrochannel; b) introducing cells in said microchannel; and c) forcingthe fluid through the fluid-resistance element such that said fluidflows through said microchannel at a flow rate.
 2. The method of claim1, wherein the fluid is forced in step c) with pressurized gas.
 3. Themethod of claim 1, wherein the fluid-resistance element comprises afluid path channeled into a substrate.
 4. The method of claim 3, whereinthe substrate of said fluid-resistance element comprises an elongatedfluid path, the first fluidic resistance being created by the elongatedfluid path.
 5. The method of claim 4, wherein the elongated fluid pathundergoes multiple windings so as to create said elongated fluid path.6. The method of claim 1, further comprising a cartridge interfaced withsaid microfluidic device. The method of claim 1, wherein the gas issubstantially insoluble in the fluid.
 8. The method of claim 1, whereinthe gas is a mixture of gases, the mixture including a gas that issubstantially insoluble in the fluid.
 9. The method of claim 1, whereinthe fluid-resistance element is removably connected to the microchannelbetween the microfluidic device and the fluid reservoir.
 10. A method,comprising: a) providing i) a microfluidic device having a firstmicrochannel, ii) a fluid reservoir comprising a working fluid and beingdisposed outside of the microfluidic device, iii) a fluid line coupledto the fluid reservoir for delivering the working fluid to or from themicrofluidic device; and iv) a fluid-resistance element coupled to themicrofluidic device, the fluid-resistance element having a first fluidicresistance that is about 10-50,000 times greater than a second fluidicresistance associated with the microfluidic device; and b) forcing theworking fluid through the fluid-resistance element such that saidworking fluid flows through said microchannel at a flow rate.
 11. Themethod of claim 10, wherein the working fluid comprises at least onecomponent selected from the group consisting of media, suspensions ofcells, particulates, proteins, chemicals, contaminant, pollutant,pharmaceutical and combinations thereof.
 12. The method of claim 10,wherein said fluid-resistance element comprises a substrate having anelongated fluid path.
 13. The method of claim 10, wherein the workingfluid is forced in step d) with pressurized gas.